common/hash_u5: routines to hash 5 bit values.

Needed for creating/verifying signatures on bolt11 invoices.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

common/hash_u5.c

@@ -0,0 +1,47 @@
+#include <ccan/endian/endian.h>
+#include <common/hash_u5.h>
+#include <string.h>
+
+void hash_u5_init(struct hash_u5 *hu5, const char *hrp)
+{
+        hu5->buf = 0;
+        hu5->num_bits = 0;
+        sha256_init(&hu5->hash);
+        sha256_update(&hu5->hash, hrp, strlen(hrp));
+}
+
+void hash_u5(struct hash_u5 *hu5, const u8 *u5, size_t len)
+{
+        size_t bits = len * 5;
+
+        while (bits) {
+                size_t n = 5;
+
+                if (bits < n)
+                        n = bits;
+
+                hu5->buf <<= n;
+                hu5->buf |= (*u5 >> (5-n));
+                bits -= n;
+                hu5->num_bits += n;
+
+                if (n == 5)
+                        u5++;
+
+                if (hu5->num_bits >= 32) {
+                        be32 be32 = cpu_to_be32(hu5->buf >> (hu5->num_bits-32));
+                        sha256_update(&hu5->hash, &be32, sizeof(be32));
+                        hu5->num_bits -= 32;
+                }
+        }
+}
+
+void hash_u5_done(struct hash_u5 *hu5, struct sha256 *res)
+{
+        if (hu5->num_bits) {
+                be32 be32 = cpu_to_be32(hu5->buf << (32 - hu5->num_bits));
+
+                sha256_update(&hu5->hash, &be32, (hu5->num_bits + 7) / 8);
+        }
+        sha256_done(&hu5->hash, res);
+}